BOSTON -- The future of a new Fenway Park may hinge on the interpretation of a single phrase in the 23-page financing package approved last month: "furtherance of a public purpose."

If the courts agree that building a new ballpark in the crowded Boston neighborhood is truly in the public interest, it could undercut the best chance opponents have of blocking construction.

But if the courts instead decide that allowing the government to seize private land to hand it over to the Boston Red Sox is an abuse of its eminent domain powers, the entire project could be in jeopardy.

Critics of the plan, who have pledged to go to court if the proposal is approved by the Boston City Council, are pinning their hopes on the second scenario.

At issue is whether a new ballpark serves enough of a "public purpose" to warrant the use of eminent domain -- the power of government to seize private land for public projects as long as the landowner is reimbursed.

The bill signed by Gov. Paul Cellucci last week lists tangible and intangible benefits, from "public pride and spirit" to "economic health" -- including a boost in hotel, restaurant, recreation, entertainment and retail sales activities.

But simply stating that a new ballpark serves a public purpose isn't enough, opponents say.

"The Legislature can say that the moon is made of blue cheese, but that doesn't make it so," said Herbert Gleason, an attorney working with Fenway opponents.

Gleason has had some experience with ballpark legislation.

In 1969, as a lawyer with the administration of former Boston Mayor Kevin White, Gleason tried to shepherd legislation that would have enabled the Massachusetts Turnpike Authority to build a new Red Sox ballpark.

That bill included an even more elaborate "public purpose" argument but was ruled unconstitutional by the Supreme Judicial Court, in part because it would have required the taking of private land, Gleason said.

Since then, the SJC has not been asked to rule on an eminent domain case in the context of a sports facility.

Earlier this year, however, Superior Court Judge Constance Sweeney cited the 1969 ruling to strike down a plan to build a proposed $21 million baseball stadium in Springfield that also would have required land takings.

Supporters of the Fenway plan argue that the Springfield decision was specific to that particular case and would not apply to Fenway. They say other cities have successfully -- and legally -- used eminent domain to clear land for new sports arenas.

Mark Bourbeau, a Boston attorney specializing in eminent domain law, agreed. Bourbeau has represented the state in land takings associated with the construction of the new underground Central Artery.

Some of the land used to build the FleetCenter was initially taken by the city through eminent domain for the "Big Dig" and then later transferred, Bourbeau said.

In recent years, the notion of a "public purpose" has become viewed more broadly than just for roads and schools, Bourbeau said. Just because the Red Sox might benefit does not detract from that public purpose provided that the private benefit is secondary to the public good.

"Courts are very leery to question the motives of legislators," he said. "The courts would have to have some evidence that there is deliberate malfeasance."

Foes of the plan are not convinced the courts will bend to the will of the Legislature so easily.

"There is no legislative language you can write that will declare something a public purpose that is not a public purpose," said Rob Sargent of MassPIRG.

The Fenway financing package enacted by the Legislature calls on the state to spend $100 million on roads and subway improvements and the city of Boston to spend $140 million on site preparation, including land takings, and $72 million on a new parking garage. The Red Sox will spend $352 on a new 44,000-seat ballpark.

The plan still must be approved by the Boston City Council.